Flight Report - PELTI Research Flight, 9 July, 2000

Considerable haze. The PRIDE group reports optical depths of 0.2 - 0.3 due
to dust. The lower mixed layer was pretty shallow, perhaps less than
1000'. There was a gradual decrease in dew point up to 6500' where there
was a sharp dryout. We would choose to fly a FT leg in the bottom of this
dry layer.

1515At 10,000' and started descent to 7500'

1519Start of eastbound leg at 7500'

Several samplers were not started at the beginning of the leg, but all
were functioning by halfway through. (An extra pump installed to achieve
isokinetic flow for the LTI froze and threw the breakers that were
powering those devices.) The LTI total flow was about 92% of isokinetic,
based on the LFEs (the thermal mass flowmeters suggest it was a bit
closer to isokinetic), and it was kept laminar the whole flight.

As the APS comparison above shows, the LTI consistently passed more
supermicron particles during this FT sample leg than the CAI and the NASA
solid diffuser. (The latter operated similarly to the CAI, but its APS
began to drift down in concentration later in the flight, so its data is
not shown for the next leg.) Some of this is no doubt due to the
enhancement of the largest particles by the curving streamlines in the
LTI, but much is no doubt due to the loss of particles in the CAI and the
solid diffuser. The DU group will do Fluent modeling to compute the
enhancement vs size in the LTI, while the UH group will make lab
measurements (monodisperse uranine particles from a VOAG) to determine the
size-dependent losses in the tubing behind the LTI and the other inlets.
The bump may be the combination of enhancements and tubing losses, both of
which are quantifiable.

1619End of 7500' leg, sounding to 100'

1635Start of 100' westbound leg

We flew this MBL leg at 220 knots IAS (rather than the usual 200 knots)
because the attack angle is under 2 deg at 220 knots. We had learned on
the ferry from Miami to St. Croix (STX, our first ever tests at 1 Atm)
that the turbulent wake from the forward CAI strut impacts the LTI inlet
at attack angles above about 2 deg. Flying a bit faster levels the plane
and allows the LTI to miss that wake. This strut is a piece that will not
be used on later projects if the LTI makes the CAI dispensable.

The wind speeds on this low-level leg were between 7 and 9 m/s, causing a
few whitecaps to be visible. This meant that there was measurable sea salt
for us to use as a test aerosol. As the figure below shows, the
large-particle difference between the LTI and the CAI was greater in the
MBL than in the FT. This could be because dry mineral particles have a
significant probability of bouncing off the inner inlet walls and being
re-entrained into the flow, while sea salt particles that impact on the
walls never come off. Clarke and Howell had inferred a similar behavior in
the CAI by comparing CAINE-2 results with INDOEX dust-layer data that was
constrained by Valero and Bucholtz's AOD profile across the layer.

At both altitudes, however, it is clear that the particle concentrations
above 2-3 um are definitely greater in the LTI sample flow. We have noted
this same behavior on all our ferry flights as well.

1715Terminated inlet comparison but remained at 100'

To gauge the significance of the observed differences in APS data, we ran
all the APSs on the same sample flow, which happens to come from the CAI.
The APS on the NASA solid diffuser inlet is clearly low, since it began
losing sensitivity during the earlier MBL sample. This may be due to a
flow rate change that has not yet been diagnosed.

1745End of 100' leg

1801Touchdown at STX

A comparison between the LTI APS and NCAR's wing-mounted FSSP 300 looks
quite promising, but is not yet vetted enough for broadcast. The
comparison between that 300 and the internal one, however, is not so
clear-cut. The 300's will be exchanged for the next flight (probably 11
July, since RAF crew duty limits require a down day) to see if a
calibration difference is responsible for the curious results.

All filters and TAS extracts are being sent off for analysis, which may or
may not be completed prior to the end of the deployment. With the
exception of Clarke's FSSP-300, most equipment is functioning well. The
total nephelometers on all inlets gave sensible results on the common
sample comparison, but the submicron one from the LTI read an order of
magnitude high and seems to be malfunctioning. In general, the LTI neph
records higher scattering than the other two when behind their respective
inlets.